In Li-ion batteries, electrodes are prepared by preparing a slurry in a solvent such as N-methyl pyrrolidone (NMP) of the anode or cathode active material particles with conductive additives such as carbon and a binder such as polyvinylidene fluoride (PVDF). The slurry is coated onto a current collector foil and dried, creating a composite porous laminate with good electronic conductivity. The electrodes are used to make a cell in which the electrolyte is absorbed into the pores of the electrode laminate. Thus the active material particles are both in electrical contact with the current collector to provide a path for the electrons produced or consumed during cell discharge and charge, and physical contact with the electrolyte to provide a continuous ionic path for Li-ions to diffuse between the anode and cathode electrodes as the cell is cycled. If a particular particle becomes fully or partially electronically isolated from the body of the conductive electrode laminate its contribution to the reversible capacity of the cell will diminish or be lost. The loss of electronic contact due to expansion and contraction of the particles also manifests itself in an excessively large loss of capacity on the first cycle, when the greatest change in electrode active material volume occurs. On subsequent cycles, the electronic isolation of multiple particles leads to an overall increase in the cell impedance and a loss of reversible capacity as more and more of the active material is isolated. In typical Li-ion cells this is a key aging mechanism eventually leading to cell failure. The process is accelerated by deep discharge cycling as the active anode or cathode particles expand and contract naturally as Li-ions move in and out of their structures. It is also accelerated at elevated temperatures where the binder can absorb the electrolyte to form gel like material that can flow, leading to separation of the active particles from the conductive additive matrix. Mitigation of this failure mechanism is critical to the development and manufacture of Li-ion batteries that can survive the thousands of cycles and years of life required to meet the demands of emerging applications in the automotive, military, energy transmission and telecommunications markets.
To date, efforts to address electrode active material conductivity issues have included coating the active materials with carbon layers. However, while increasing the inherent electronic conductivity of the active material particle, it does not solve the problem of maintaining contact with the laminate conductive matrix. Such particles can still become isolated as the cell is cycled or aged.